China
Africa
Explore chapters and articles related to this topic
Attosecond Metrology
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Pierre Agostini, Andrew J. Piper, Louis F. DiMauro
The concept of the streak camera is well known: a photocathode converts the photon pulse into an electron pulse which is swept by a voltage ramp and detected on a 2D detector. The width of the image carries information on the pulse duration, assuming knowledge of the initial image width (ideally a thin slit) and of the streak speed. A typical streak camera time resolution is ps. The translation into the attosecond regime is obtained by using the IR electric field sub-cycle time dependence as a streaking ramp. The photoelectron replica of the XUV attosecond pulse is analysed in a spectrometer as a function of the delay, and the final momentum distribution allows to retrieve the pulse duration.
Functional Optical Brain Imaging
Published in Hualou Liang, Joseph D. Bronzino, Donald R. Peterson, Biosignal Processing, 2012
A wide variety of both commercial and custom-built fNIR instruments are currently in use [5]. These systems differ with respect to their use and system engineering, with tradeoffs between light sources, detectors, and instrument electronics that result in tradeoffs in the information available for analysis, safety, and cost. Three distinct types of fNIR implementation have been developed: time domain (TD) systems, frequency domain (FD) systems, and continuous wave (CW) spectroscopy systems, each with their own strengths and limitations [3–5]. In TD systems, also referred to as time-resolved spectroscopy (TRS), extremely short (picosecond-order) incident pulses of laser light are applied to the tissue and the temporal distribution of photons that carry the information about tissue scattering and absorption is measured. The emerging intensity is detected as a function of time (the temporal point spread function [TPSF]) with picosecond resolution [7,39,41]. Streak camera systems can provide high-temporal resolution, but they are large, expensive, and have a limited dynamic range. Time-correlated single-photon counting systems can be built with cheaper components and can provide wide dynamic range, however, they have poor temporal resolution. Even though TRS instruments offer absolute measurements of hemodynamic changes since they can be large, expensive, with limited dynamic range or poor temporal resolution, they have originally been developed as laboratory-based devices, and hence are difficult to be implemented in the clinical environment and in the field applications [5]. In FD or phase modulation spectroscopy (PM) systems, the light source is intensity modulated to the frequencies in the order of tens to hundreds of megahertz. The amplitude decay, phase shift, and modulation depth of the detected light intensity with respect to the incident light are measured to characterize the optical properties of tissue [42]. FD methods are low-cost alternatives to time-resolved methods and hence several multichannel FD instruments are now in common use [7,43].
Efficient x-ray lasers in neonlike ions utilizing transient gain
Published in S Svanberg, C-G Wahlström, X-ray Lasers 1996, 2020
P V Nickles, V N Shlyaptsev, M Schniirer, M Kalachnikov, T Schlegel, W Sandner
The experiments were performed at the recently upgraded CPA Nd: glass laser facility at the Max-Born-Institute, Berlin [16]. The laser delivers two synchronized laser beams with different pulse durations. A Ti:Sapphire start oscillator, followed by a stretcher and a regenerative amplifier (Spectra-Physics), are used as a 1053 nm, 1 ns (FMHW) front-end system for a linear glass amplifier chain. Before entering the power amplifier the stretched pulse is splitted into two beams, then being amplified seperatelv. One pulse is recompressed to than 0.7 ps, the second one is kept at 1 ns duration. The corresponding maximum pulse energies were 4 J and 7 J. The two beams are polarized orthogonally and are switched with a polarizer into the same axis in front of the cylindrical focusing optics. The cylindrical double lens focussing optics enables a line focus on the target with a width of 30 μm and an adjustable length of the focal line. Typical lengths of 1 to 5 mm have been used in experiments. The x-ray emission from the plasma was recorded by an on axis transmission grating spectrograph consisting of a toroidal mirror at grazing incidence and a free-standing diffraction grating (2000 lines/mm). The design is similar to that reported in [17]. The Ni-coated toroidal mirror produces a 1:1 image of the plasma emission on the recording plane. To get time-resolved spectrograms a x-ray streak-camera (Kentech) with a CCD system was coupled to the spectrograph. The spectrograph had an acceptance angle of 15 mrad in both directions. Both the dispersion direction and the detection slit of the streak-eamera‘s photocathode were orientated perpendicular to the plane of incidence of the pump-laser beam. Home designed photocathodes (100 nm Csl on 300 nm Al, supported by a 2000 lines per inch Ni-mesh) for the wavelength range of λ > 17 nm have been used in the streak-camera. The main advantage of this Al=foil-Csl cathode consists of a high absorption for wavelengths shorter than the Al L-absorption edge at 17 nm, providing a good suppression of second order grating diffraction below 34 nm. The temporal resolution of the streak camera was ≤ 10 ps. The spectral resolution of the wThole recording system was about 0.5 nm.
Study on the shot-to-shot fluctuation of intensity and spectra of femtosecond pulses in seeded low-gain free-electron laser at DELTA
Published in Journal of Modern Optics, 2020
At DELTA, a femtosecond pulse train with ∼4 mJ per pulse, 1 kHz repetition rate and 40 fs width from a Ti:sapphire laser (for details see ref. (53)) co-propagates with the electron bunches (∼100 ps, rms) through the modulator (called U250) to imprint a coherent energy modulation at the scale of the laser wavelength on a fraction of about 10−4 of the bunch. The M2 factor of the laser beam is measured to be M2 ≈ 1.32 (53). In Figure 1, the spectral and temporal characteristics of the seed laser after amplification is presented. It is noticeable that the spectral bandwidth, Δλ = 30 nm, experienced spectral narrowing because of the gain narrowing phenomena. It is a consequence of the fact that the centre region of the optical spectrum experienced a higher gain than the spectral wings due to inhomogeneous gain distribution. The inset shows the measured pulse duration of 41.2 fs of the compressed laser pulses with SPIDER after two-stage amplification. The laser pulses are focused on the centre of the modulator by a telescope. The focal spot size of the laser at the centre of the modulator is measured to be in the order of the electron beam spot size about 500 μm. The temporal overlap of the laser pulses and the electron bunches is achieved by monitoring the undulator radiation and seed laser with a streak camera. The spatial overlap is obtained by comparing the position of the undulator radiation and seed laser on a screen. The delay between laser pulses and electron bunches is adjusted with a vector-modulator at the DELTA SR.